Pool Troubleshooting /
What Causes Algae in a Pool?
Algae usually starts when chlorine is too low for the water’s demand, or when circulation and filtration can’t distribute sanitizer well enough to keep growth from taking hold.
Archive date: this article was written for PoolSkimmer on Oct 23, 2026 and given the earlier archive date of Sep 18, 2026 so the blog reads as an organized archive. It was not published on that date.
The source of algae in a pool
For the most part, algae is a sign that the water balance or sanitizer has failed to keep up. It can be as simple as chlorine levels being too low for what the water requires, or circulation and filtration not doing their job of moving the sanitizer around. While you may have a lot of swimmers, rain or organic matter putting a load on the pool, the root cause is usually that the chemistry is no longer in check.
Treated water and algae
You might wonder why algae makes an appearance when the water appears to be in order. The test results do not always tell the whole story regarding how much active sanitizer is at hand. When chlorine and stabilizer are not in sync, the pool can give the impression it is fine until it goes cloudy or green.
Some of the usual suspects are:
- Maintenance that was put off or chlorine not added
- A lot of swimmer activity driving up demand
- Runoff from a storm or rain
- Leaves and dirt
- Filtration that is weak, leaving sanitizer unevenly spread
The impact of filtration and circulation
In and of themselves, these systems will not create an algae problem, but poor operation can make one difficult to put down. If the pump and filter are not up to snuff, debris lingers and sanitizer does not get where it needs to.
A good way to diagnose the situation is to:
1. Put the water through its paces for free chlorine, pH and stabilizer. 2. Make sure the pump and filter are running as they should. 3. Give the walls, ladders and steps a brush to break up any buildup. 4. Get rid of any floating leaves. 5. Adhere to the chemical label and let the circulation run as called for during the cleanup.
pH and the role of the sanitizer
Chlorine is your primary line of defense against algae; pH is what determines how well it does its job. Should the pH wander out of range, the water will not sanitize properly even with chlorine in it.
There are some rules to follow:
- Do not assume high pH is just the debris’s fault.
- Never mix chemicals.
- Rely on the product label and equipment manual rather than making assumptions.
- Have the right PPE on hand.
First steps when you see algae
Before getting into the details of treatment, address the obvious:
1. Run a test. Verify your chlorine, pH and stabilizer numbers. 2. Check the circulation. Confirm normal operation of the filter and pump. 3. Brush and skim. Clear the surfaces and remove debris. 4. Empty the skimmer basket. For safety, shut the pump down, take the lid off and lift out the basket to empty it. Put everything back in place before you turn the pump back on. 5. Keep the pool closed if you cannot verify safe conditions. Swimming is not advised while you are cleaning up.
Time to call in a pro?
If the filter and pump are not behaving, the water remains cloudy after you have done your work, or the algae is persistent, it is best to engage a qualified pool professional. This also applies if there is any question of an electrical issue, a damaged drain cover or a suction hazard.
A word on safety
- No mixing of products.
- Gloves and goggles are a must when you are handling chemicals.
- Do not put anything into operating suction equipment or near an outlet.
- Turn the pump off to get at the skimmer basket.
- Follow the manuals and labels without exception.
- While you are in the midst of a chemical treatment or algae cleanup, the pool should be kept closed.
Sources and further reading
- [HASA: Green pool causes and cleanup](https://hasa.com/blog/why-is-my-pool-green-how-to-clean-and-prevent-algae-naturally)
- [CDC: Keeping your pool safe and healthy](https://www.cdc.gov/healthy-swimming/safety/what-you-can-do-to-stay-healthy-in-swimming-pools.html)
- [Pentair U-3 skimmer manual: suction safety warnings](https://www.pentair.com/content/dam/extranet/nam/pentair-pool/pool-manuals/u-3-skimmers/395010028C.pdf)
- [SwimmingPool.com: Cleaning the skimmer basket](https://www.swimmingpool.com/maintenance/pump&filter/cleaning-the-skimmer&pump-basket)
- [CDC: Use pool chemicals safely](https://www.cdc.gov/healthy-swimming/media/pdfs/Pool-Chemical-Safety-USE-poster-p.pdf)
- [CPSC: Electrical safety in and around pools](https://www.cpsc.gov/safety-education/safety-guides/pools-and-spas/dont-swim-shocks-electrical-safety-and-around-pools)